Magnetic readers are used in disc and tape drives for reading changes in the magnetic field of charges stored on a magnetic storage medium, such as a magnetic disk or tape. The read head comprised by the magnetic reader can be one of several types. Currently, three basic types of read heads are in common use, namely, composite heads, which are comprised of a ferrite core with a wire coil and a glass-filled gap, metal-in-gap (MIG) heads, which comprise a ferrite head in which metal alloys are sputtered into the magnetic gap of the head, thin-film heads, which comprise coil windings deposited on a ceramic base using semiconductor fabrication techniques, and magneto-resistive (MR) heads, which comprises a nickel-iron magneto-resistor sensing element which is fabricated using semiconductor fabrication techniques.
Composite heads, MIG heads and thin-film heads all utilize inductive elements that sense the rate of change of the magnetic field on the surface of the storage medium as the head passes over the surface of the storage medium. The change in magnetic flux energizes the inductive element, which results in the inductive element producing an analog output voltage signal. This output voltage signal, commonly referred to as the read-back signal, is typically filtered, amplified and converted into a digital representation of the information stored on the storage medium.
MR heads operate differently from these other types of common heads. MR heads operate in accordance with a phenomenon known as the magneto-resistive effect. Generally, this effect is characterized by a change in resistance of the magneto-resistor when the magneto resistor senses a change in the magnetic flux on the surface of the storage medium. The resistance of the magneto resistor depends on the angle of the current running through the magneto resistor as well as the direction of the magnetization of the magneto resistor. A constant sense current is passed through the magneto-resistor. As the MR head passes over an area on the surface of the storage medium having a change in magnetic flux, the resistance of the magneto resistor changes, resulting in a change in the amperage of the sense current. The voltage across the magneto resistor is kept constant Alternatively, a constant dc current may be passed through the magneto resistor and a change in voltage will occur when the resistance of the magneto resistor changes.
One advantage of the MR head over these other types of heads is that the MR head directly senses magnetic flux, whereas the other types of heads are energized by the flux. Therefore, greater flux is needed to energize the inductive elements to a level sufficient to produce an analog voltage output signal that is great enough to result in an accurate reading of the information contained on the storage medium. Therefore, the magnetic domains of the areas on the storage medium being sensed must be relatively large. The MR head, on the other hand, is capable of sensing smaller magnetic domains, which facilitates the use of smaller bit cells and narrower track widths, thus increasing the storage capacities of the storage mediums.
Another advantage of MR heads over heads that utilize inductive elements is that MR heads can be used more effectively than heads that utilize inductive elements with low-velocity magnetic tape and disk drives. Since the inductive elements detect rates of change of the magnetic flux, the velocity of the storage medium will affect the read-back signal. For disc drives with lower velocities, the read-back signal will not be as strong as it normally is with high-velocity drives and, therefore, more filtering and amplification of the read-back signal may be required. The weaker read-back signal may also result in a relatively low signal-to-noise (SNR) ratio. In contrast, since the magneto resistor senses magnetic flux directly, MR heads are relatively insensitive to the velocity of the storage medium. Therefore, they can be used with lower-velocity disk and tape drives while having relatively high signal-to-noise ratios.
The resistance of the magneto resistor is related to the angle between the direction of the current flowing through the magneto resistor and the direction of magnetization of the magneto resistor. The relationship between the resistance of the magneto resistor and this angle is characterized by a cosine squared curve. The cosine squared curve that defines this relationship is relatively linear when the angle is 45.degree.. Therefore, in order to maximize SNR, it is desirable to cause the magneto resistor to operate in this linear range. Various schemes have been developed to bias the direction of magnetization away from the direction of the current flow in order to cause the magneto resistor to operate in this linear range. However, biasing the magnetization results in many other problems that must be dealt with, such as, for example, the formation of magnetic domains in the magneto resistor which obfuscate the angle of magnification of the magneto resistor.
The cosine squared curve is also relatively linear when the polarity of the current is reversed such that the direction of the current is away from the direction of magnetization, while still at an angle of 45.degree. with respect to the magnetization vector. An MR read head normally works better (i.e., SNR is more satisfactory) when biased in one of these directions than it does when it is biased in the opposite direction. Typically, an MR read head is designed to operate in the linear range of the cosine squared curve with the bias current directed in the direction that is expected to maximize performance of the read head. The MR head read amplifier is then designed to bias the MR element in the chosen direction.
It would be beneficial to provide a head read amplifier that is capable of reversing the polarity of the bias current in order to bias the MR element in either of these two directions. One of the advantages of designing the head read amplifier in this manner is that the read head could be tested to determine which polarity maximizes performance and then the polarity that resulted in the best performance could be used to bias the MR element. Such a feature could result in increased head yield by making an otherwise unusable read head satisfactory. This is particularly important with respect to a multi-head drive in which as many as ten read heads may be connected on the same substrate. In this case, if any one of the read heads is faulty, the entire multi-head assembly must be replaced.
Another advantage of designing the head read amplifier to bias the MR element in either direction is that the head read amplifier could be used with read heads that need to be biased in different directions. Thus, the head read amplifier could be used with read heads purchased from different sources that have opposite biasing requirements.
However, providing a head read amplifier which is capable of switching the polarity of the bias current creates noise problems in the head read amplifier that must be overcome. Accordingly, a need exists for a method and apparatus for switching the polarity of the bias current in an MR head read amplifier which allow the polarity of the bias current to be switched without degrading the performance of the head read amplifier.